1. Field of the Invention
The present invention relates to a lighting device of a metal halide lamp noticed as a new light source for a vehicle, and more particularly relates to a lighting device of a discharge lamp which lengthens the life of a metal halide lamp.
2. Description of the Prior Art
In the field of automobiles, individuality, safety or environmental adaptation is strongly demanded. In order to meet such demand, pursuit of design of a car body in highly individual style and pursuit of improvement of traveling safety are being carried out. For example, with respect to a headlight, in place of an incandescent lamp as usual, adoption of a tungsten is becoming popular. Further, in order to improve the light quality of a headlight, and at the same time in order to meet demand with respect to style, adoption of a metal halide lamp is being studied.
A metal halide lamp is a sort of a high intensity lamp (HID). Among various artificial light sources, since the metal halide lamp is close to the sunlight (good in color rendering) in comparison with any other lamps and moreover can generate electric energy to be consumed as light efficiently (good in the light source efficiency), it is called the ultimate lamp.
FIG. 1 is a schematic diagram showing structure of a metal halide lamp. The metal halide lamp 100 has a quartz tube 101 sealed at both ends, and a light emitting tube 102 enclosed at the center of the quartz tube 101. The light emitting tube 102 is provided with tungsten electrodes 103a, 103b, which are connected through molybdenum leaves 104a, 104b to external leads 105a, 105b. Metal iodide 106 made of several sorts of metals such as sodium, scandium reacting with iodine, starting gas (e.g., xenon) 107 and mercury 108 are filled into the light emitting tube.
Light emission of the metal halide lamp is carried out as follows. First, starting discharge occurs by the starting gas 107. Next, when the inside of the light emitting tube becomes high temperature and high pressure, generation of arc due to the starting gas 107 occurs. The metal iodide having been evaporation and in gaseous form, enters the arc, and is then separated into metal ions and halogen ions. That is, a state occurs where current is transmitted. Soon the temperature at the center portion of the arc attains to about 4,500 K., and the metal ions emit light strongly within the arc. At the same time, of locations where arc is not generated, metal and halogen are recombined into metal iodide 106. The ionization and the recombination are repeated as above described, thereby the strong light having inherent spectrum of the metal atom is continued to be emitted. Such a lighting device for a vehicle using a metal halide lamp is disclosed, for example, in JPA No. 8299/1991.
FIG. 2 is a block diagram showing an example of a lighting device of a high voltage discharge lamp for a vehicle in the prior art. In FIG. 2, numeral 1 designates a battery, numeral 16 designates a DC step-up circuit with an input terminal connected through a lighting switch 15 to the power source terminal of the battery 1, and numeral 17 designates a high-frequency step-up circuit provided at the output side of the DC step-up circuit 16 for converting the battery voltage into sine wave AC voltage. For example, an inverter circuit in pushpull system is used as the high-frequency step-up circuit 17. Numeral 18 designates a current limiting load and igniter circuit, and a discharge lamp 9 is connected to the output end of the circuit 18.
Numeral 19 designates an igniter starting circuit for sending starting signals to the current limiting load and igniter circuit 18, and numeral 20 designates a control circuit. Numerals 21, 22 designate resistors provided for voltage dividing between output terminals of the DC step-up circuit 16, numeral 23 designates a variable resistor provided between the resistors 21, 22, and numeral 24 designates a sensing resistor provided on one connection line connecting the DC step-up circuit 16 and the high-frequency step-up circuit 17.
The control circuit 20 generates a pulse signal of duty cycle in response to output voltage of the DC step-up circuit 16 detected through the resistors 21, 22 and the variable resistor 23 and voltage from the sensing resistor 24. Then the pulse signal is sent to the DC step-up circuit 16, and the output voltage of the DC step-up circuit 16 is controlled.
Next, the operation will be described. When the lighting switch 15 is turned on, a starting pulse is generated by a signal sent from the igniter starting circuit 19 to the current limiting load and igniter circuit 18. The starting pulse is applied between the first and second electrodes 9a, 9b of the discharge lamp 9. The step-up control of the battery voltage is carried out at any time by the control circuit 20, and finally the transfer to the steady state of the lamp is accomplished.
Since the lighting device of high pressure lamp for vehicles in the prior art is constituted as above described, for example, when the high-frequency step-up circuit 17 is a pushpull inverter, it follows that the discharge lamp 9 is lit in plus or minus potential as shown in FIG. 3.
When the discharge lamp 9 is lit, metal atoms filled into the light emitting tube of the discharge lamp 9 are ionized into ions with plus charge. When the surrounding of the light emitting tube is at lower potential than that by the plus charge, the metal ions are drawn toward quartz (SiO.sub.2) of the light emitting tube. Most of various metal ions are obstructed by the barrier of the quartz glass. However, since only the sodium ion (Na.sup.+) is liable to react with quartz, the sodium ion penetrates into the quartz glass and finally passes through it. This is a phenomenon known as so-called Na loss.
Consequently, when the surrounding of the discharge lamp 9 is at lower potential than the mean lighting potential, due to the Na loss as above described, sodium spectrum (589 nm) is gradually decreased from light spectrum emitted by the discharge lamp. Therefore the red color becomes pale from the color of the emitted light. The metal halide lamp originally has the excellent color rendering and the chromaticity. However, a problem exists in that these properties are significantly deteriorated due to the Na loss and life of the discharge lamp 9 becomes short extremely.
Particularly, when a lighting tool and its periphery are connected to ground as in a lighting device of high pressure lamp for vehicles, probability for the above-mentioned problem to occur is very high. Nevertheless, such problem has never been considered.
This problem occurs not only in the case of using the high-frequency step-up circuit 17 by a pushpull inverter but also in the case of using the high-frequency step-up circuit 17 by a half bridge, a full bridge or the like.